After trialling with the club during the 2011 MLS pre-season, Farfan signed with Philadelphia Union on March 18, 2011 He made his professional debut the following day, in Philadelphia's opening game of the 2011 MLS season, a 1-0 win over Houston Dynamo. Farfan scored his first professional goal on May 28, 2011, in a comprehensive 6-2 victory over Toronto FC.